This invention relates to polychromators and more particularly to a polychromator apparatus and technique for adapting the image plane to the detector plane and for attaining optimal linearity of the wavelength scale in the plane of the detector.
In a polychromator, a spectrum is generated in the plane of a detector by a dispersing element, e.g. a grating. The spectrum is formed by the images of an inlet slit generated by different wavelengths. The detector is a "locally resolving detector" which simultaneously detects the radiation at different points in the plane of the spectrum. Such a locally resolving detector may be a photographic plate which is blackened in accordance with the light intensity of the different spectral lines. Such a detector may also be a diode array that is a sequence of photodiodes closely arranged side by side. In the photodiode detector, the different components of the spectrum are simultaneously imaged on different photodiodes such that the different wavelengths of the spectrum are parallelly detected.
In such a polychromator, the detector plane is generally flat. Therefore, the image plane composed of the monochromatic images of the inlet slit should be as flat as possible. This is particularly true for the tangential section while the sagital section is less critical because the width of the slit determining the resolution becomes effective in the tangential section. Furthermore, it is desirable to obtain a wavelength scale as linear as possible on the detector so that the distance of a monochromatic slit image measured transversely to the direction of the slit is linearly dependent on the wavelength.
In known polychromators having a concave grating, the concave grating forms the only optical element which images the inlet slit on the plane of the detector. The concave grating also thereby simultaneously provides for the spectral splitting up of the images of the inlet slit. Thus, the concave grating simultaneously provides two functions: It provides an image of the inlet slit and it serves as a dispersing element.
Furthermore, it is known to optimize such polychromators in the tangential section with regard to the image plane such that the image plane coincides as well as possible with the plane of the detector. The optimization therein is made such that the slit image lies exactly in the plane of the detector with three wavelengths within the utilized spectral range. The remaining defocussing is minimum at the other wavelengths. However, with such an optimization of the image plane, a nonlinear arrangement of the slit images associated with the different wavelengths results. The distance of the different monochromatic slit images from a reference mark measured transversely to the direction of the slit depends nonlinearly on the wavelength. It can be attempted to optimize the linearity of the wavelength scale in a similar manner, but then the optimal adaptation of the image plane to the plane of the detector is not achieved.
Accordingly, it is an object of the present invention to provide a new and improved polychromator.
Another object of the invention to provide a polychromator which simultaneously achieves adaptation of the image plane to the plane of the detector in the tangential section and optimal linearity of the wavelength scale on the plane of the detector.
Another object of the invention is to provide a technique for optimizing the image plane characteristics and wavelength scale characteristic of a polychromator.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
It has been found that the foregoing and related objects and advantages are attained in a polychromator having an inlet slit for introducing a beam of light from a light source, a concave grating for spectrally dispersing light from the inlet slit into a wavelength spectrum, and a locally resolving detector for simultaneously detecting radiation at different points in a plane. An imaging mirror is disposed along the path of rays for deflecting the spectrally dispersed light from the grating so as to image a wavelength spectrum of slit images on the detector. In a preferred embodiment of the invention, the path of rays in tangent section extends substantially z-shaped from the inlet slit to the detector. The concave grating is angularly oriented relative to the mirror by reference to the white light position of the grating relative to the mirror. Preferably, the axis is normal to the plane of the grating is oriented toward the mirror relative to the white light position of the normal axis. With this configuration, the image plane of the slit images formed by the mirror is adapted to the flat detection plane of the detector at the wavelengths of the spectrum and the wavelength scale along the detection plane has optimal linearity.
Thus, it has been found that optimization of the image plane and optimization of the wavelength scale can be achieved at the same time by using an additional imaging mirror in the special configuration of the present invention.
In the method of the present invention, the image plane of the mirror is optimumly adapted to the plane of the detector while simultaneously attaining an optimum linear wavelength scale on the detector by alternate variation of the distance between the concave grating and the mirror and the asymmetry measure G' of the concave grating in converging steps.